1. Field of the Invention
The present invention relates generally to thermosetting compositions that contain copolymers of vinyl monomers. More specifically, the present invention is directed to a co-reactable solid particulate mixture thermosetting composition that includes functional copolymers containing isobutylene type monomers and a monocarboxylic acid.
2. Description of Related Art
Reducing the environmental impact of coating compositions, in particular that associated with emissions into the air of volatile organics during their use, has been an area of ongoing investigation and development in recent years. Accordingly, interest in powder coatings has been increasing due, in part, to their inherently low volatile organic content (VOC), which significantly reduces air emissions during the application process. While both thermoplastic and thermoset coating compositions are commercially available, thermoset coatings are typically more desirable because of their superior physical properties, e.g., hardness and solvent resistance.
Low VOC coatings are particularly desirable in the automotive original equipment manufacture (OEM) market due to the relatively large volume of coatings that are used. However, in addition to the requirement of low VOC levels, automotive manufacturers have very strict performance requirements of the coatings that are used. For example, automotive OEM clear top coats are typically required to have a combination of good exterior durability, acid etch and water spot resistance, and excellent gloss and appearance. While liquid top coats containing, for example, capped polyisocyanate and polyol components, can provide such properties, they have the undesirable drawback of higher VOC levels relative to powder coatings that have essentially zero VOC levels.
Coating compositions containing polyol and capped polyisocyanate components (“isocyanate cured coatings”) are known and have been developed for use in a number of applications, such as industrial and automotive OEM topcoats. Such isocyanate cured coating compositions are described in, for example, U.S. Pat. Nos. 4,997,900, 5,439,896, 5,508,337, 5,554,692, and 5,777,061. However, their use has been limited due to deficiencies in, for example, flow, appearance, and storage stability. Isocyanate cured coating compositions typically include a crosslinker having two or more capped isocyanate groups, e.g., a trimer of 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane capped with e-caprolactam, and a hydroxy functional polymer, e.g., an acrylic copolymer prepared in part from a hydroxyalkyl acrylate and/or methacrylate.
Solid particulate coating formulations referred to in the industry as “powder coatings” are finding increasing use in painting metal substrates, both as primer coatings and as top or clear coats of the base coat and clear coat composite coatings. Conventional powder coating compositions can contain a variety of polymeric film-forming materials with functional groups including, but not limited to, polyacrylates, polymethacrylates, polyesters, polyamides, polyethers, polysilanes, and polysiloxanes. In certain instances, polycarboxylic acid functional crosslinking agents, such as for example, dicarboxylic acids, are commonly used in the powder coating compositions. One example of a dicarboxylic acid commonly used in powder coating compositions is dodecanedioic acid. Such dicarboxylic acids are typically more costly than monocarboxylic acids.
The automotive industry provides corrosion protection and decorative appearance for motor vehicles by multilayered paint composites on the various types of surfaces of motor vehicles. The finishing layers of this composite usually include the popular base coat clear coat composites. The base coat is a pigmented layer and the clear coat is a nonpigmented or only slightly pigmented layer that is applied separately to the base coat and cured to some degree with the base coat. To provide improved coating composites for motor vehicles, the industry is seeking solutions to the problem of blemishes, smudges, and coating imperfections that occur due to the action of acid rain and road dirt, and debris that may strike areas of the vehicle, as well as car wash brushes, which can result in unaesthetic marring of the clear coat. Mar resistance is the property of a coating film which enables the film to remain unimpaired by light abrasion, impact, or pressure.
It would be desirable to develop thermosetting “powder coating” compositions that include functional copolymers and a monocarboxylic acid that provide excellent acid resistance and mar resistance at low VOC levels and can have good appearance and low cost.